This invention relates to thermoplastic elastomers made by the reaction of a thiol terminated polysulfide resin with an epoxy resin wherein the ratio of thiol equivalents to epoxy equivalents in the epoxy resin is critical to the formation of the thermoplastic elastomer rather than a liquid, a solid that is too soft and weak to be useful, or a solid that cannot be softened and processed at a temperature below its decomposition point. The invention here also relates to hot melt adhesives and sealants, particularly hot melt sealants for insulated glass windows.
The use of thiol terminated liquid polysulfides in caulks, sealants and the like for numerous applications is well known. Such materials normally are applied as liquids and chemically cured to solids in situ. Although the uncured polysulfides are classified along with most other natural and synthetic rubbers as thermoplastic materials, they have also been considered as sharing the property of most rubbers that once vulcanized, they cannot be induced to flow under heat and pressure without extensive degradation of the basic polymer chain and the consequent permanent loss of most or all of the desired properties. As is well known, also, it is difficult to hit a target average molecular weight and, thus, a target viscosity, when making liquid polysulfides. The molecular weight is usually adjusted by oxidation/reduction processes that are not completely efficient.
Liquid polyepoxide resins are typically cured by reaction with a second liquid resin such as an amino resin to give a hard, durable product. These two part liquid systems are widely used in adhesives, coatings and the like but for certain purposes a flexibilizer resin must be added to reduce rigidity.
The invention described in U.S. Pat. No. 2,789,958 formed the basis for the use of polysulfides as flexibilizers for epoxy resins. The reaction between the thiol groups of the polysulfides and the epoxy groups of epoxy resins in the presence of an amine catalyst gave hard, tough, sometimes rubbery products. When the catalyst was omitted, a tough rubbery polymer was obtained. In either case, the product was a thermoset elastomer.
Others in the field have followed up on the teachings of U.S. Pat. No. 2,789,958 but in every instance noted, the epoxy resin/polysulfide resin adduct has either been a liquid or a hard thermoset resin. U.S. Pat. Nos. 3,363,026; 3,536,656; 3,732,309; 4,214,067; and 4,689,389, along with British Patent 787,022, are representative of these efforts.
Hot melt sealants, in general, have not exhibited good structural properties on cooling and thus have not been adequate in applications where structural support by the sealant is required as in insulated glass window sandwiches. U.S. Pat. Nos. 4,165,425 and 4,314,920 teach the preparation of hot melt sealants from thiol terminated liquid polysulfides by extrusion. The present invention provides the convenience of the hot melt application and the good structural properties of a thermoplastic elastomer.
Thermoplastic elastomers (or TPE's) are materials that can be extruded, blow molded, injection molded, or otherwise processed like thermoplastics yet have properties similar to conventional rubbers. These materials offer several advantages over the conventional, thermoset, rubbers such as: simpler processing, shorter fabrication times, recyclable scrap, lower energy consumption, and better quality control. They are classified into six general categories: styrenic block copolymers; rubber-olefin blends; elastomeric alloys; thermoplastic polyurethanes; thermoplastic copolyesters; and thermoplastic polyamides. The present invention provides a new category.